Thin film metal alloy magnetic recording disks typically comprise a substrate, such as an aluminum-magnesium (AlMg) alloy with a nickel-phosphorous (NiP) surface coating, a cobalt-based alloy sputter-deposited as a magnetic layer on the substrate, and a protective overcoat, such as a sputter-deposited amorphous carbon film, formed on the magnetic layer. A general description of the structure of such thin film disks is given in Opfer et al. U.S. Pat. No. 4,610,911 and Lin et al. U.S. Pat. No. 4,552,820.
The coercivity (Hc) is an important factor in the design of such disks. Where high magnetic recording density is required, it is desirable to have a high coercivity limited only by the write capability of the recording head. In addition, it is desirable to have low intrinsic media noise so that the resulting readback signal has a high signal-to-noise ratio (SNR).
One of the problems in the manufacture of such disks is that it has not been previously possible to easily vary the coercivity of such disks to achieve a predetermined value of coercivity. One conventional approach to manufacture a disk having a selected coercivity is to vary the composition of the sputtering target. For example, Opfer, et al. U.S. Patent 4,610,911, describes varying the platinum content in a cobalt-platinum (CoPt) alloy disk so as to vary the coercivity. This method has the inherent disadvantage that it requires that the manufacturing line be shut down so that a new sputtering target can be inserted into the sputtering chamber. Another technique for selectively varying the coercivity is to use underlayers between the disk substrate and the magnetic layer, which results in an enhancement of the magnetic properties, including the coercivity, of the subsequently deposited magnetic layer. This technique is described in assignee's U.S. Pat. Nos. 4,654,276 and 4,652,499 wherein tungsten (W) and an alloy of chrominum-vanadium (CrV), respectively, are suggested as the enhancement or nucleation layers to improve the coercivity of the disks.
Reactive sputtering gases, such as nitrogen-containing and oxygen-containing gases, have been used as dopants for cobalt-based alloy disks (CoNiPt, CoPtCr, CoCrTa) to adjust the coercivity over a desired range and to attain several coercivity objectives from a single sputtering target. The doping of the cobalt-based magnetic film is achieved by sputtering the magnetic film (or both the underlayer film and the magnetic film) in a mixture of argon (Ar) and the reactive gas (e.g. O.sub.2, N.sub.2, H.sub.2 O vapor). In this regard, Yamashita, et al. U.S. Pat. No. 4,749,459 describes a sputtering process using N.sub.2, or a gas such as H.sub.2 O vapor which dissociates to release oxygen, to vary the coercivity of CoPt alloy disks. Khan, et al. in IEEE Transactions on Maqnetics, Vol. 24, No. 6, Nov. 1988 (pp. 2985-2987) teaches the use of O.sub.2, N.sub.2 and H.sub.2 O vapor to control the coercivity of CoCrTa alloy disks. The problem with the use of reactive sputtering of cobalt-based alloy films in the presence of nitrogen-containing and oxygen-containing gases is that it is not possible to predict the effect of the sputtering process on the intrinsic media noise, and thus the resulting SNR, of the completed disk.